In a conventionally known optical head for use in an optical disk unit or a magneto-optical disk unit, to the end of accurately converging a light beam upon each track on a disk, and collecting the light reflected therefrom with a light receiving element, a moveable member carrying an object lens is supported by a base member via four sheet springs at a neutral position so as to be moveable in both the focusing and tracking directions, and the moveable member is actuated in a controlled manner by using a magnetic actuator consisting of a focusing coil, tracking coils, and permanent magnet pieces.
In such an optical head, spurious vibration modes of the spring system supporting the moveable member and the angular error of the object lens with respect to the optical axis in a low frequency range tend to impair the accuracy in converging the light beam, and this could lead to read/write errors.
In particular, there has been a growing demand to increase the rotational speed of the disk in the optical disk unit or the magneto-optical disk unit, and the response speed of the head to achieve a higher access speed, and this makes the problems associated with the vibration of the moveable member in a floating support even more serious.
It is possible, as proposed in Japanese patent laid open publication No. 7-78437 for instance, to arrange the moveable member and the coils symmetrically with respect to the optical axis, and to make the point of application of the inertial force on the moveable member coincide with the point of application of the restoring force of the spring and the electronic drive force as accurately as possible so that such undesired vibrations may be minimized. However, in such an optical head which is not adapted to place the object lens at an extreme end of the moveable part, compact and light-weight design involves some difficulty. For this reason, a design which would allow the moveable member and the coils to be arranged in an asymmetric pattern is more desirable.
For instance, the arrangement proposed in Japanese patent laid open publication No. 6-325387 comprises a main body having a relatively large vertical through hole for receiving a core standing upright from a base member, and a support piece for the object lens which extends from the main body in a direction perpendicular to the focusing direction, the focusing coil and the tracking coils being retained inside the vertical through hole. However, the support piece for the object lens tends to have a reduced thickness, and the vibration of this part could lead to a problem.
For instance, Japanese patent laid open publication No. 5-197991 discloses an arrangement in which a so-called inertia damper using a spring and a mass is used for controlling the rotation in a direction perpendicular to the focusing direction. It may be conceivable to apply this arrangement to that including an object support piece extending from the main body, and use the inertia damper to directly control the vibration of the support piece. However, it is difficult to install the inertia damper without involving optical interferences. Also, the structure tends to be highly complex while the moveable part tends to be both bulky and heavy, and the freedom in design would also be restricted.
According to the object lens actuator employing a moveable member having an object lens support piece extending from the spring-supported main body in a direction perpendicular to the focusing direction which is disclosed in Japanese patent applications Nos. 8-250982 and 8-250983 (Japanese patent laid open publications Nos. 10-79128 and 10-79129) filed by the applicant of this application, an impact damper consisting solely of a spring or consisting of a spring and a mass extends from the main body so as to strike and move away from the support piece when the main body vibrates. The impact damper vibrates and strikes the support piece in an opposite phase from that of the support piece. This suppresses higher resonant frequency vibrations of the support piece, and controls the vibration of the moveable member supporting the object lens with the result that the rotational speed of the disk and the head speed may be increased. However, this arrangement requires a certain mounting space around the object lens because the impact damper is required to be adjacent to the object lens support piece, and the moveable part tends to be undesirably bulky and massive, although not so much as an inertia damper.